无法在 swift 中使用 associatedType 作为方法 returnType，因为编译器抛出“关联类型的模糊推断”

Question

Was playing with associatedType. Compiler is not recognising the return type of the method when using associatedType.

Here is the code sample,

protocol DummyOffice {
}

struct EmptyOffice: DummyOffice {
}

protocol  Office {
    associatedtype SubBranch: DummyOffice
    var subBranch: SubBranch { get }
    func getSubBranch() -> SubBranch
}

struct Apple: Office {
    let emptyOffice = EmptyOffice()
    func getSubBranch() -> some DummyOffice {
        return EmptyOffice()
    }
    
    var subBranch: some DummyOffice {
        return EmptyOffice()
    }
}

and compiler throws this error.

Questions:

(1) Error is not occurring for the property "subBranch". Thats is if I didn't create a method that returns associatedType in the protocol, everything works smooth.

Answer 1

Using an opaque return type (ie some ) is not required for what you want to do. When you go to implement the Office protocol, just return the actual type from the function and computed property you specified and the compiler will infer the associatedtype for you:

protocol DummyOffice {}

struct EmptyOffice: DummyOffice {}

protocol Office {
    associatedtype SubBranch: DummyOffice
    var subBranch: SubBranch { get }
    func getSubBranch() -> SubBranch
}

struct Apple: Office {
    let emptyOffice = EmptyOffice()
    func getSubBranch() -> EmptyOffice {
        return EmptyOffice()
    }
    
    var subBranch: EmptyOffice {
        return EmptyOffice()
    }
}

Answer 2

As @zaitsman suggested, replacing associatedTypes with typealias is working good.

But typealias cannot be used as psuedo generic constraint,

protocol DummyOffice {
}

struct EmptyOffice: DummyOffice {
}

protocol  Office {
    typealias SubBranch =  DummyOffice
    var subBranch: SubBranch { get }
    func getSubBranch() -> SubBranch
}

struct Apple: Office {
    var subBranch: Self.SubBranch
    
    func getSubBranch() -> Self.SubBranch {
        return EmptyOffice()
    }
    
}